The field of the present invention is directed in general to an ion chamber-type neutron detector, and more particularly to extended life and improved sensitivity of such detectors when used to measure the neutron flux in a nuclear reactor core.
An example of an incore neutron detector system of the type which the present invention may be employed is shown by G. R. Parkos et al. in U.S. Pat. No. 3,565,760, entitled "Nuclear Reactor Power Monitor System", which patent is hereby incorporated by reference.
Ion chamber type neutron detectors are well known and are shown for example in U.S. Pat. No. 3,043,954 by L. R. Boyd et al., entitled "Fission Chamber Assembly", which patent is hereby incorporated by reference. Usually such chambers comprise a pair of spaced electrodes electrically insulated from one another, with a neutron sensitive material and an ionizable gas therebetween. For example, in a fission type ion chamber, the neutron sensitive material is a material such as uranium 235 which is fissionable by thermal neutrons. As neutrons induce fissions of the uranium in the chamber, the resultant fission products ionize the gas in proportion to the magnitude of the neutron flux in the chamber. When a direct current voltage is applied across the electrodes, an output signal is created which is proportional to the amount of ionization and hence proportional to the neutron flux in the chamber.
Since the early days of nuclear fission reactors, neutron sensitive ion chambers have been used for control of Light Water Reactors (LWR) during startup as well as full power operation. The Power Range Monitors (PRM) for sensing neutron fluxes in LWR's operating at full power have typically been miniature fixed incore fission chambers. Full power operation is generally defined as operation of the LWR at 100% of its designed full power rating.
However, the ion chambers used for measuring the neutron flux during initial startup (i.e. the Source Range Monitors, abbreviated SRM, and the Intermediate Range Monitors, abbreviated IRM) of the LWR have not been fixed in the core of the reactor. Typically, the startup sensors include four source range monitors (SRM) which cover the neutron flux range from 10.sup.3 to 10.sup.9 nv, and eight intermediate range monitors (IRM) which can cover the range from 10.sup.8 to 1.5.times.10.sup.13 nv. The common unit of flux is defined as the number of particles crossing a unit area per unit time, and is a measure of intensity. For this discussion, the neutron flux is indicated by the symbol "nv", which denotes the number of neutrons which pass through an area of one square centimeter in one second. Together, these sensors and associated electronics cover greater than ten decades of neutron flux.
FIG. 1 is a graphical representation of the source range, intermediate range, and power range operation of a BWR, and the various responses of the three different sensors to various levels of neutron flux.
Because of the need for high sensitivity in the source range and intermediate range, and in order to prevent premature burnup of the SRMs and IRMs during full power operation of the LWR, the SRM and IRM sensors have heretofore been retracted to a position below the BWR core where the neutron flux is negligible. The system for inserting and retracting the sensors consists of drive control electronics, drive motors, flexible drive shafts, gear boxes, and vertical drive tubes which contain the sensors and provide a means for inserting them into a hollow cylindrical dry tube fixed in and extending into the core of the BWR. These components require a high level of maintenance, are subject to damaqe during control rod drive maintenance, and add to under-vessel clutter.
Another problem associated with the high level of maintenance required for the retractable detectors is that human beings are exposed to radiation. As is known, the Nuclear Regulatory Commission (NRC) sets an upper maximum of radiation to which a human being can be exposed for a given period of time, typically referred to as the man-rem exposure.
Additionally, the NRC has published a new regulation in Regulatory Guide 1.97, requiring operating plants to have a safety related system to monitor neutron flux levels after a loss-of-coolant accident (LOCA) from a power range of 10.sup.-6 percent all the way up to 100% of full power.
Therefore, a need exists for a sensor which (1) can be fixed in the reactor core without rapid burnup, (2) can remain in the reactor core for three or more operating cycles or approximately 5 full power years in a BWR, and (3) operates over a wide range of neutron fluxes preferably with the use of one sensor in place of the presently used two sensors.